In most of vortex flowmeters based on the theory that the number of Karman's vortexes generated per unit time by a vortex generator mounted in a passage of the fluid flowing in a flow pipe is substantially proportional to a flow rate of fluid, heat-sensitive elements are used for detecting a Karman's vortex street because it is compact and cheap. Typical heat-sensitive elements may be, for example, a heat wire, a metal foil and a thermistor. It is connected to and heated by a constant voltage or constant current power source. A fluctuation of a flow of fluid with a vortex street generated therein causes the heated heat-sensitive element to radiate and varies its resistance. A variation of resistance of the heat-sensitive element is converted through a bridge circuit into a current or voltage signal. The measured signal can have a specified voltage or current value if the measurement was conducted with the same fluid at the same temperature and the same flow rate. However, the measurement of the same fluid may have variations if the fluid varied its temperature and/or included dirty particles causing the sensing element to vary its radiation coefficient. The latter is irrevocable with a heat-sensitive element working in direct contact with the fluid. The above-mentioned problem may be solved by a known method of detecting a variation in a stream of clean purge gas such as air and nitrogen gas, which can vary its resistance with a change of a fluctuating vortex pressure of the fluid to be measured. Namely, the sensing element can measure a stream of clean purge gas in place of fluid to be measured. This enables the sensing element to indirectly measure the flow rate of dirty or high-temperature or low-temperature fluids which cannot be measured by the conventional method.
However, a vortex signal having a high S/N ratio cannot be obtained only by the introduction of a purge stream to convert a vortex signal to a purge signal. A conventional method to solve the above problem was described in Japanese Patent Publication No. 62-13606, which method is to detect a vortex signal with a high S/N ratio by a substitution method by feeding a constant purge stream of inert gas (such as nitrogen, and air) from an inert-gas supply source disposed outside the flow meter. This method, however, requires the use of an expensive flow meter capable of measuring and monitoring a purge flow to obtain vortex signals having high S/N ratios over a wide range of measurement of flow rates. Furthermore, the above-described method has such a disadvantage that a flow rate of purge gas must be accurately adjusted by means of a control valve to a specified value allowing the flowmeter to reliably measure the purge flow even if the fluid pressure of the fluid changes.
To solve the above-mentioned drawbacks of the conventional method, the present applicant proposed a vortexflow meter in Japanese Utility Model Publication No. 5-47380, which flowmeter is intended to operate with no need of conducting adjustment of the purge flow and no need of using an expensive device. This was achieved by using a critical purge stream generated by a throttling means operating on the condition that a ratio of a purge gas pressure to a pressure at an open end of each conduit. Namely, the purge flow can be always maintained at a constant value independent of pressure variations of fluid to be measured. The flowmeter can thus be constructed with use of a cheap orifice plate and a critical nozzle as the throttling means: the throttling means is mounted in a middle position of a conduit tube with a vortex sensing element, into which a constant flow of purge gas is fed.
FIGS. 1A and 1B shows an exemplified construction of a conventional purge type vortex flowmeter which corresponds to the vortex flowmeter described in Japanese Utility Mode Publication No. 5-47380. The purge type vortex flowmeter shown in FIG. 1A includes a vortex generator 2 mounted in a flow pipe 1 and conduits 4a and 4b (normally represented by a conduit 4) penetrating into the inside of the flow pipe 1 through the wall thereof and having pressure ports 3a and 3b in neighborhood of the both corresponding sides of the vortex generator 2. Purge streams alternatively fluctuate and flow out through the pressure ports 3a and 3b of the conduits 4 by the effect of the fluctuating vortex pressure. The fluctuations of the purge flow in the conduits 4a and 4b are detected by sensing elements 7a and 7b. 
The sensing elements 7a and 7b may be heat-sensitive elements such as for example heat wires and thermistors. The heat-sensitive elements 7a and 7b to be used in contact with a purge stream are disposed between paired throttling elements 6a and 6b arranged in the middle of the conduit 4 (in the boundary portion between the conduits 4a and 4b). These heat-sensitive elements 7a and 7b form respective arms of a bridge circuit (not shown). The paired throttling elements are slightly separated from each other to form a laminar flow in the middle portion of the conduit 4 to remove a noise component. A stream of purge gas (air or nitrogen) from the high-pressure inert gas supply source is supplied to the middle portion of the conduit 4 and detected by the heat-sensitive elements 7a and 7b disposed therein. Detection signals from the heat-sensitive elements 7a and 7b are transferred through lead wires to an amplifier 9 whereby they are amplified. The amplified detection signals are cleaned off noise components by a filter circuit and then outputted as respective vortex signals.
In order to maintain the constant supply of the purge flow, a throttling element 20 is mounted in an upstream purge tube (small-diameter tube) 5 connected to the middle portion of the conduit 4 in which the heat-sensitive elements 7a and 7b are disposed. An example of an orifice plate 201 serving as the throttling element 20 is shown in FIG. 1B. This throttling element 20 has a small diameter (d1) inlet 21 and a large diameter (d2) outlet 22 and a purge gas stream flows in the direction shown by an arrow. Let assume that a purge stream has a pressure P2 at the outlet and a pressure P1 at the inlet of the throttling element 20. When a ratio of the outlet pressure P2 to the inlet pressure P1 is increased equal to and grater than the critical pressure ratio, the purge stream from the outlet 22 obtains a velocity equal to the sound velocity and becomes a well-known constant mass flow free from the influence of pressure variations of the purge stream on the downstream side. More precisely, the flow rate of the outlet purge stream may vary with variations of pressure, temperature and humidity of the inlet side purge stream but the variations of the outlet stream may be negligible and does not cause any problem in regard to the accuracy of the purge flowrate. The purge flowrate can be changed by changing the area (diameter d1) of the inlet 21. Since the critical pressure ratio P2/P1 of purge air is equal to 0.528, a pressure reducing valve 11 may be adjusted so that the pressure P1 of supply purge air indicated on a pressure gauge 12 may be high enough to allow the throttling element 20 to obtain the critical ratio of the supply gas pressure P1 to the purge air pressure P2 to be measured. Even if variation of the gas pressure P1 to be measured occurred, a constant purge stream can be supplied into the system as far as the above-mentioned condition is maintained.
As described above, the vortex flowmeter disclosed in Japanese Utility Model Publication No. 5-47380 can introduce an accurate purge stream by using merely a simple and cheap throttling element such as for example an orifice plate (without applying any expensive flowmeter and flow control elements such as valves) and eliminates the need of adjusting valves even with a change in the gas pressure to be measured (with no need of using pressure control means).
FIGS. 2A and 2B shows another exemplified construction of conventional purge type vortex flowmeter which differs in construction from the flowmeter of FIGS. 1A and 1B by arrangement of purge conduit tubes 4 and positions of exhaust ports serving as pressure ports. In this purge type vortex flowmeter, a vortex generator 2 is mounted in a passage of fluid flowing in a flow pipe 1 and conduits 4a and 4b extend through the vortex generator 2 to respective external side surfaces thereof whereat they communicates at their pressure ports 3a and 3b with the fluid passage in the flow pipe 1. In the same way as described for the flowmeter of FIGS. 1A and 1B, this vortex flowmeter can supply clean purge fluid such as nitrogen gas from the external purge gas source through the respective conduits into the passage along the both sides of the vortex generator 2 and detect alternative changes of the purge streams by the effect of Karman's vortexes produced by the vortex generator by using flow velocity sensors such as thermistors. Since the detecting sensors can work not in direct contact with the fluid to be measured, this flowmeter can measure a flow rate of dirty fluid or high-temperature fluid or low-temperature fluid which could not be measured by the conventional flowmeters.
However, the quality of detection signals obtained by the sensors depends on a flowrate of purge gas flowing along the working surfaces of the sensors. Namely, the larger a flow of fluid flowing in the flow pipe (the larger an alternating differential karman's vortex pressure is), the larger a purge flow is required by the sensor portion. On the contrary, the smaller a flow of fluid flowing in the flow pipe (the smaller an alternating differential Karman's vortex pressure is), the smaller a purge flow is required by the sensor portion. To always obtain high-quality sensor signals, it is necessary to control the flowrate of the purge gas in accordance with the flowrate of the fluid to be measured flowing in the flow pipe. As described above, the vortex flowmeters are featured by supplying a constant flow of the purge gas to the sensors. However, it cannot control in practice the purge gas flow in response to a change in flowrate of the fluid in the flow pipe. As the result of this, the stabilized and high-quality detection signals cannot be obtained, thereby the number of vortexes produced per unit time can not accurately measured.